1. Field of the Invention
The present invention relates to materials which reduce the filtration of fluid into permeable earth formations during cementing processes in the drilling and completing of subterranean wells, particularly wells for the recovery of petroleum resources.
Petroleum well cementing is the process of mixing a slurry of cement, water, and other additives and pumping it down through steel casing to critical points in the oil well annulus around the casing or in the open hole below the casing string. The primary functions of the cementing process are to restrict fluid movement between geological formations and to bond and support the casing. In addition the cement aids in protecting the casing from corrosion, preventing blowouts by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells, and sealing off lost circulation or thief zones.
A common problem in petroleum well cementing is the flow of liquid from the cement slurry into porous earth formations in contact with the cement. This fluid loss is undesirable since it can result in dehydration of the cement slurry, and it causes thick filter cakes of cement solids which can plug the well bore. The fluid lost can damage sensitive formations. Cement fluid loss is particularly a problem in the process known as squeeze cementing.
There is a requirement, therefore, for materials which, when added to the cement formulation, reduce the loss of fluid from the slurry to porous formations.
2. Description of the Prior Art
A type of fluid loss agent used in oil well cementing consists of a medium molecular weight grade of hydroxyethylcellulose (HEC) which may be employed with or without a dispersant such as condensed napthalenesulfonic acid salts. There are several disadvantages to the use of hydroxyethylcellulose as a cement fluid loss control agent, however, among which are that it causes undesirable viscosification and retardation of the cement and that it loses effectiveness in the presence of soluble calcium salts and at elevated temperatures.
Another cement additive employed for control of fluid loss is a copolymer of acrylamide and acrylic acid [L. F. McKenzie, F. M. McElflesh, SPE 1-623,279 (1982)]. Although this material performs well at high temperature and in the presence of soluble calcium salts, it has the undesirable property of strongly retarding cement. This retarding effect increases with increasing temperature, most likely due to the hydrolysis of the unstable amide groups contained in the polymer which accelerates at elevated temperature. Hydrolysis of amide functions produces additional carboxylic acid residues which are credited with cement retarding activity.
Still other materials utilized for the control of fluid loss during well cementing are combinations of polyamines and either lignosulfonate or condensed naphthalenesulfonic acid salts [L. F. McKenzie, J. V. Eckerts, and P. M. McElfresh, Oil and Gas J. 80 (13) 146 (1982)]. These additives are valuable since they cause little viscosification of the cement, and they have little sensitivity to dissolved calcium and moderately high temperatures. The utility of the polyamine-sulfonate combination is somewhat limited, however, by two side effects they produce: retardation and settlement (free water) of the cement.
Mixtures of HEC, polyvinyl pyrrolidone and sodium naphthalenesulfonate have been reported as cement fluid loss additives (Wersent, U.S. Pat. No. 3,132,693 (1964)) as have mixtures of poly(vinylpyrrolidone) and sodium naphthalenesulfonate-formaldehyde condensation products (U.S. Pat. No. 3,359,225).
Also complex mixtures of maleic anhydride-N-vinylpyrrolidone copolymers with polymers of poly(aryl-vinylbenzyl)alkyl- and hydroalkyl substituted quaternary ammonium bases and salts have been used (Wahl, U.S. Pat. No. 3,140,269 (1964)).
An improved fluid loss additive for well cements consists of 30 to 70 weight percent of N-vinylpyrrolidone homopolymer, from about 5 to 19 weight percent of carboxylmethylhydroxyethylcellulose, and the sodium salt of condensed naphthalenesulfonic acid. [B. W. Hale, U.S. Pat. No. 4,258,790 (Mar. 31, 1982)].
Copolymers of N-vinylpyrrolidone and metal salts of styrenesulfonic acid (1) are known, but only as components in oil containing microcapsules used in films. [K. Saeki, H. Matsukawa, U.S. Pat. No. 3,855,146 (1974)].
Despite the art for the control of cement fluid loss which is already known, there exists a need for novel agents capable of improving fluid loss control which are not reduced in utility by the limitations described above.